Safety and performance are important considerations in a drilling riser. With trends over the past decades to exploit resources in deeper waters and harsher environments, ensuring the safety and performance of drilling risers has become a challenging task.
A riser tensioning system aims to compensate for relative motions between a floating drilling rig and the seabed, which are joined by a rigid riser string. In conventional systems, the most widely used riser tensioning system is a hydro-pneumatic riser tensioning system consisting of hydro-pneumatic cylinders, air/oil accumulators, and air pressure vessels. However, there are short-comings in hydro-pneumatic tensioning systems.
First, the response time for a hydro-pneumatic tensioning system is too slow for certain situations. The relatively slow operation of pneumatic systems results in a long control response time, which is the time between issuing a command and force being applied by the tension system. In certain situations, such as during an emergency riser disconnect, the tension changing response may be too slow. The slow, large over-pulling force may accelerate free riser pipes outward, allowing them to jump out, and consequently damage the drilling rig floor and riser pipes.
Second, increasing longitudinal over-pull tension, the conventional method in hydro-pneumatic tensioning systems used to suppress destructive vortex-induced vibration (VIV), causes stress on the supporting equipment, increases wear and tear on the tensioning system, and increases riser pipe fatigue. Furthermore, increasing longitudinal over-pull tension raises safety concerns in situations where a pair of hydro-pneumatic tensioners are receiving maintenance while the drilling rig is experiencing high wave conditions.
Third, a hydro-pneumatic tensioning system is a relatively complex and costly system that requires a significant amount of maintenance and is at risk for hydraulic fluid leakage. A hydro-pneumatic tensioning system includes a hydro-pneumatic cylinder rod and a seal that are exposed to bending due to factors such as vortex-induced vibration (VIV) or unequal and non-linear loading caused by vessel roll and pitch. These factors may cause high failure risk and may require a high maintenance cost to avoid hydraulic fluid leakage and risks of environmental pollution. Furthermore, the complex hydro-pneumatic system includes a significant volume of air accumulators and reservoirs that consume useful floor space on a drilling rig.